Change of electronic state and crystal structure by postannealing in superconducting SrFe2(As0.65P0.35)2

Abstract

We investigated the annealing effects on the physical properties of ${\mathrm{SrFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2(x=0.35)$. The superconducting transition temperature ($T_c$) increased from 26 to 33 K by annealing. The x-ray diffraction measurement suggested that the annealed crystals have the shorter/longer $a/c$ axes and the larger pnictogen height $h_{\mathrm{Pn}}$. This must be linked to the $T_c$ enhancement by annealing. Moreover, it was found that the postannealing decreased the electronic specific heat coefficient at $T=0\mathrm{K},{\gamma }_r$, and changed the magnetic field ($H$) dependence from sublinear ${\gamma }_r\propto H^{0.7}$ to $H$ linear ${\gamma }_r\propto H$. This can be attributed the electronic change from dirty to clean superconductors with a $s_{\pm }$ gap.